Communication systems are directed to systems for demodulating and decoding according to the signal strength or its induced value, and include wired and wireless communication systems, and broadcasting and data accessing communication systems. In the typical communication systems, the signal strengths in different channels rise and fall due to the influence of the characteristics of the channels. If the strength of the received signal of the receiving end is adjusted by an automatic gain controller, the received signal may be kept within a fixed strength range to prevent the receiving end from misestimating the message of the received signal.
In the practical system, however, the signal strength adjusted by the automatic gain controller is not perfectly unchanged as in the assumption mentioned above. The operation principles of the automatic gain apparatus in the Gaussian channel and the fixed multi-path channel environment will be described by taking the Gaussian channel environment as an example. In the Gaussian channel environment, the automatic gain controller adjusts the value of the loop gain, and makes a balanced consideration between the gain converging speed and the gain jitter extent in the stable state. When the loop gain is high, the speed of converging the gain to the ideal level is fast, but the up and down jitters above and below the ideal level become severe, as shown by the up and down jitters of the gain curve 100 above and below the ideal level 10 after the time point t1 of FIG. 1. When the loop gain is low, if the switching point is not good, the speed of converging the gain to the ideal level is slow, but the up and down jitters above and below the ideal level become smooth, as shown by the dashed line 110 or 120 of the gain variation of FIG. 1.
Thus, the system designers have proposed various methods of adjusting the gains to improve the gain converging speed and the condition of the jitters of gains in the stable state. For example, a method characterized in the algorithm of the gain adjusting control signal generator is disclosed in U.S. Pat. No. 7,336,743. The method includes course and fine regulation stages, and adjusts the gain in a stepped manner in the course stage to improve the gain converging speed. The course method is to determine to increase the gain, decrease the gain or keep the gain unchanged according to the result of the signal strength comparator, wherein the increased or decreased value of the gain is a predetermined constant. In addition, U.S. Pat. No. 7,295,073 discloses a method characterized in that the gain is adjusted according to the maximum amplitude of the signal so that the signal sampled in the range of the analog-to-digital converter has the maximum utilization efficiency. The method integrates the signal strength estimating function and the signal strength comparing function into the amplitude level detector, and the detected result is outputted to the range detector. The range detector determines to increase the gain, decrease the gain or keep the gain unchanged according to the result of the amplitude level detector, wherein the increased or decreased value of the gain is a predetermined constant.
In the dynamic channel environment, however, when the stable track signal strength of the automatic gain controller changes and immediately compensates the path loss, the adjustment signal strength is continuously adjusted in the code period time, thereby deteriorating the receiving performance of the receiver by the additionally introduced jitter noise. For example, the conventional automatic gain controller, such as that disclosed in each of U.S. Pat. Nos. 7,336,743; 7,295,073; 7,031,409 and 6,574,292, periodically adjusts the gain at the system working frequency. In the dynamic environment, the additionally introduced jitter noise deteriorates the receiving performance of the receiver, so it is greatly influenced by the non-linear noise and the dynamic channel.
To sum up, the conventional automatic gain controller, which is made in order to improve the gain converging speed, cannot effectively avoid the problem that the noise is introduced when the track path loss changes. Thus, the noise greatly influences its receiving performance in the dynamic environment.